Many modern electronic systems function, at least in part, in accordance with digital arithmetic principles. While some systems, often those involving the greatest complexity, utilize so-called parallel arithmetic, an important class of data presented for digital arithmetic processing is serial in nature. Serial arithmetic is characterized by the operands being a time sequence of bits and the result being a similar bit stream. For example, serial addition consists of the two summand bit streams feeding a 1-bit adder, least significant bit first. The carry output of the 1-bit adder is fed back and used as an input to the adder, along with the next input bit, and the required sequence of sum bits is the resultant output bit stream. Further, it is well known that a serial-parallel pipeline multiplier can be constructed with a set of 1-bit adders. See, e.g., Freeny, "Special-Purpose Hardware for Digital Filtering", Proceedings of the IEEE, Vol. 63, No. 4, April 1975, pp. 633-648.
Serial arithmetic has found application in digital signal processing as, for example, in the serial addition of a pair of digitally encoded, serially transmitted voice channels to establish a conference call. Another typical application of serial arithmetic is in digital filtering, where both addition and multiplication are required. The digital signal processing environment is often a real-time system in which high speed and low power are desired. The high speed is desired to obtain a high multiplexing factor that will reduce the cost per operation, while the low power is desirable to reduce the operating costs and to increase the packaging density.
A great variety of circuits has been designed for implementing 1-bit binary full adders. A useful summary of optional designs using standard AND/OR and NOR/NAND gates is given in Liu et al, "Optimal One-Bit Full Adders With Different Types of Gates," IEEE Transactions on Computers, Vol. C-23, No. 1, January 1974, pp. 63-70.
Logic circuits using current-switching circuit techniques and, in particular, series gating circuit techniques have long been known in the electronic arts. Recent advances in large scale integrated transistor circuits have suggested the possible application of current-switching logic circuits for realizing, among other things, special purpose arithmetic circuits such as adders and multipliers.
For example, such an adder is marketed by Motorola Semiconductor Products, Inc., under the designations MC1019 and MC1219, as described more particularly in MECL Integrated Circuits Data Book, Motorola, Inc., 1973. Current-switching logic circuitry is attractive for a number of reasons, many of which are pointed out in the literature. It has a very low power-delay product, with the delay being primarily a function of a single RC time constant. Therefore, the power-delay product can be maintained over a range of speeds. The relatively slow RC rise times and the balanced nature of current-mode circuits result in low internal noise levels. Also, the wired OR, wired AND, and other standard techniques of current-mode logic permit the realization of specified logical functions with a minimum of circuitry.